x86: remove usage of CONFIG_KERNEL_LINK_IN_VIRT

There is no need to use this kconfig, as the phys-to-virt
offset is enough to figure out if the kernel is linked in
virtual address space in gen_mmu.py.

For code, use Z_VM_KERNEL instead.

Signed-off-by: Daniel Leung <daniel.leung@intel.com>